The present invention relates generally to aerosol and certain gaseous element filters and samplers and, more particularly, to filter-holding apparatus which permits the filter to be preloaded, and thereby readily changed, and which will allow vapor-phase materials to be sampled simultaneously with particulates. This invention was made with government support under Contract No. W-7405-ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
A variety of sampling instrumentation exists for extracting a sample of airborne materials for measurement. Common among such instrumentation is the requirement for introducing a filter element in the sampled airstream such that the aerosol and vapor-phase constituents are removed and deposited on the filter or captured in an absorber. In many applications, the physical characteristics of the filter medium are critical to the success of the sampling process (i.e., pore size, physical structure, chemical formulation, etc.). Sampling media are frequently fragile, requiring mechanical support during the sampling process sufficient to prevent rupture due to the large pressure differences generated by the sampling pump, and requiring careful mounting and sealing to prevent misalignment, tearing, or creasing which could allow sampled air to bypass the filter altogether. Other concerns arise from operator handling of filters in the field during the exchange process, such as cross-contamination of samples or exposure of the operator to toxic materials. The requirement for careful mounting of some filter media leads to increased operator time to complete filter exchange, and difficulties achieving automation of the filter exchange process.
Continuous air monitoring (CAM) instrumentation is widely used in the nuclear industry to monitor and detect the presence of radioactivity in workplace air and stack-discharge air. Typically, CAM devices monitoring for particulate radioactivity in air collect aerosol particles on a substrate medium, which is a filter through which the continuous sample is drawn. Radioactive emissions from the collected sample are detected by a planar detection device (solid state or gas-filled chamber) placed parallel to the collector surface and separated by a small gap through which the air sample must pass.
There are several aspects of the alignment of the sample filter, the sample flow path, and the detector-filter gap space, which affect the performance of the CAM instrument, especially in the situation where the accurate determination of the energy of the radioactive emissions from the sample is critical to the detection and alarm functions. First, the filter medium must be consistently and accurately positioned in line with the detector. Failure to accurately and repeatably position the sample-collection surface with respect to the detector can result in changing geometry for radiation detection, thereby affecting detection efficiency. Moreover, loss in energy discrimination due to differing energy absorption from sample to sample may also occur. Second, the design of the flow path from the CAM inlet to the filter can affect the quantity of the sample lost on the internal surfaces of the CAM head. If these deposits occur in positions from which irradiation of the detector can occur, spurious signals can result, leading to inaccurate measurements or false alarms. Hence, sample flow path can affect both the sampling efficiency and accuracy of the instrument. These undesirable deposits can also contaminate the filter-holding assemblies resulting in possible subsequent contamination of the operator, and possible cross-contamination between samples. Third, the gap between the filter surface and the detector must be repeatably and accurately maintained to ensure accurate energy determination and efficiency.
In addition to repeatable, accurate radiation detection, there are several additional CAM head design considerations related to the filter-holding and positioning mechanisms which are critical. First, since many filter media are fragile, the method of mounting the filter in a holder must not result in a twisting or tearing of the medium as closure is achieved. Moreover, the seal must be sufficiently tight and positive that leaks around edges and joints which would allow a portion of the sampled air volume to bypass the filter do not occur. Second, the filter-exchange process should be ergonomically efficient so that a manual change of filter can be reliably and rapidly made. There should be a minimum of wasted motion and possibilities for inadvertent loss of the samples, or cross-contamination. Mechanical assembly/disassembly associated with the filter-change process should not cause rapid wear of instrument components or surfaces leading to premature failure or frequent service and repair.
In the past, a distinction has been drawn between the interpretation of the results of CAM sampling and fixed-air sampling (FAS). This is because it was assumed that the sample obtained by a CAM is not representative of the size distribution of the contaminant aerosol, and therefore, of the concentration near the point of release, due to the sampling characteristics of the CAM inlet (and sampling line), and to the placement of the CAM. The FAS sampler, by contrast, is generally placed closer to potential release points, and has no sampling inlet bias. With the development of more recent CAM technology, this distinction is not as clearly justified as it once was. Furthermore, modern CAM heads have the capability of being located remotely from the data processing and display unit.
All CAMs must have means for introducing a sample collection medium (filter) into, and removing it from, the sample airstream adjacent to a detector. The short range of some of the radiations of interest in CAM applications dictates that the filter be placed in close proximity to the detector, so that during the filter-exchange procedure, the detector and filter must be moved apart such that the filter holder can be opened and a replacement filter installed. A "clam-shell" design, which was found in some early CAM designs, allows the filter holder to be rotated away from the detector while maintaining the vacuum connection to the holder. All elements of the filter-holding apparatus remain attached to the instrument. A disadvantage of this design is that the aerosol-flow pattern in the "clam-shell" tends to impinge upon the holder, thereby resulting in inadvertent aerosol loss and contamination of extraneous surfaces. Moreover, the precut filter medium itself must be carefully handled by the operator during the filter-change procedure in order to avoid damage and cross-contamination. Other designs permit the complete removal of the filter-holding apparatus from the CAM head during filter change. While this can simplify the filter-handling operation, it can create another difficulty, since now the filter holder is separated from the vacuum source, which connection must be reestablished. One manufacturer uses a drawer to move a filter in and out of their alpha-emitter particulate CAM head. Aerosol enters the head through a large-diameter entrance nozzle and is directed into the aerosol-inlet plenum which surrounds both the filter holder in the drawer and the detector, thereby being exposed to extraneous surfaces in its path to the filter. The entire aerosol-inlet plenum is at negative pressure (a vacuum) relative to ambient pressure, necessitating a vacuum seal at both the interface with the vacuum connection at the base of the drawer, and the outside case of the head near the drawer handle. In the former instance, the seal is required to be a sliding seal, subject to wear and tear. The filter is held in place on the drawer with a "filter ring holder", a thin steel ring, which clamps the filter down by action of a magnet in the drawer. Change of filter in the ring-and-magnet case is difficult and time-consuming, and is subject to cross-contamination due to the requirement for handling, removing, and replacing the filter medium and the ring holder.
Accordingly, it is an object of the present invention to provide a filter cartridge which gives sufficient support to the filter material such that rupture thereof is prevented as a result of pressure differentials applied thereto.
Another object of the present invention is to provide an aerosol filter, or a combination aerosol filter and absorber, cartridge which permits accurate alignment of the filter with a detector and long-term reliable sealing of the cartridge/filter medium combination so that sampled gas cannot bypass the filter.
Yet another object of the invention is to provide a filter cartridge which reduces operator exposure to dangerous materials deposited on the filter cartridge and filter medium and cross-contamination thereof by the operator.
Still another object of the present invention is to provide a simple, low-cost filter cartridge which can be preloaded by the operator with a fresh filter medium, under controlled laboratory conditions, so that the filter changing procedure in the field involves replacement of the filter cartridge. This is in place of replacing the filter medium itself, and is adaptable to automatic introduction and removal from air sampling devices.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.